Today most of a local telephone company network is the local subscriber loop, i.e., the loop from a central office (CO) to subscriber equipment such as a landline telephone. The local subscriber loop is presently being used to provide broadband digital communication services such as digital subscriber line (DSL) service. Such broadband DSL services include integrated services digital subscriber network (ISDN), high-rate digital subscriber line (HDSL), asymmetrical digital subscriber lines (ADSL) and very high rate digital subscriber lines (VDSL) technology. DSL services allow subscribers to send and receive digital data at higher rates of speed than were previously possible using analog modem technology.
DSL technology exploits existing metallic telephone loop plants to provide megabit per second (Mbps) high-speed Internet access and other services and the great majority of subscribers are served by such metallic (copper) twisted pair cables connected from a local switch in the CO to the subscriber equipment, for allowing telephone and DSL signals to travel on the cables.
There are many impairments to DSL transmission including loop loss and crosstalk, as DSL signals are attenuated and distorted by transmission through the loop, particularly at high frequencies and on loops with bridged tap. Some of the power of a DSL transmitting on a loop travels through a crosstalk-coupling path and generates crosstalk noise into other DSLs on loops in the same cable. Additional impairments include electromagnetic interference due to radio ingress, which appears as narrowband noise spikes in the frequency domain, and impulse noise which occurs as brief spikes in the time domain. However, crosstalk that generally increases with increasing frequency often becomes the major limitation to high-speed DSL.
If a DSL or other system transmits with a power spectral density (PSD) on one pair of a multi-pair cable, resulting crosstalk couples into a nearby pair, whereas the crosstalk can be calculated by multiplying the transmit PSD with a crosstalk coupling function or crosstalk gain function in the frequency domain. In this context, the so called “spectral compatibility” is the property that crosstalk between different systems that transmit in the same twisted-pair cable does not significantly degrade the performance of any of the systems. Spectrum management is the process of deploying DSLs in the loop plant (communication network) in such a manner that ensures spectral compatibility. During this management, spectrum balancing algorithms are used to optimize PSD for the DSLs to find suitable operating points, i.e. conventional parameters for controlling the DSLs.
Current techniques for spectrum management apply relatively rigid rules uniformly across the entire loop plant, as embodied in ANSI T1.417, the Spectrum Management Standard developed by ANSI-accredited DSL standards committee TIE1.4. For example, these rules do not take into account the individual types of crosstalk sources and crosstalk couplings of a particular cable, which may be considerably different than the worst-case couplings that are assumed in the spectrum management standard. Dynamic Spectrum Management (DSM) technology intends to consider scenario dependent conditions and requirements, such as cable properties, power consumption and system performance. Current DSM techniques, such as SCALE (Successive Convex Approximation for Low-complexity), try to optimize operational parameters for a specific set of constraints and requirements. The existence of a solution is not known in advance and a solution is isolated in that it is valid for the assumed constraints and requirements only.
Other techniques related to spectrum management is described in U.S. Pat. No. 7,106,833, U.S. Pat. No. 7,151,803 and U.S. Pat. No. 7,158,563.